<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        img {
            width: 31em;
            position: fixed;
            top: 50vh;left: 50vw;
            transform: translate(-50%, -50%);
            
        }

        body.dimmed::before {
            content: '';
            position: fixed;
            top: 0;right: 0;
            bottom: 0;left: 0;
            z-index: -1;
            background: rgba(0, 0, 0, .8);
        }
    </style>
</head>
<body class="dimmed">
    <img src="dog.jpg">
    <p>1</p>
    <p>1</p>
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    <p>1</p>
    <p>1</p>
    <p>1</p>
    <p>1</p>
    <p>1</p>
    <p>1</p>
    <p>1</p>
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    <p>1</p>
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    <p>1</p>
    <p>1</p>
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    <p>1</p>
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    <p>1</p>
    
</body>
</html>